Automatic frequency control circuit



Sept. 6, 19.38. D, E;l FOSTER l 2,128,997l

I AUTOMATIC FREQUENCY CONTROL CIRCUIT Filed April 10, 1937 mme fmwTUN/N6 RANGE EQU/mmf /NnucrAA/cs 0F was /2 Patented Sept. 6, 1938AUTOMATIC FREQUENCY CONTROL CIRC UIT

Dudley E. Foster, South Orange, N. J., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application April 10, 1937, SerialNo. 136,064

7 Claims.

My present invention relates to automatic frequency control circuits forradio receivers of the superheterodyne type, and more particularly to auniformly acting frequency control network for the local oscillator tankcircuit of a superheterodyne receiver.

At the present time automatic frequency control circuits (AFC) forsuperheterodyne receivers generally comprise a discriminator unit forderiving a direct current voltage from the intermediate frequency (IF)energy when the latter shifts in frequency from the assigned IF value.In addition to the discriminator unit, there is l utilized a frequencycontrol tube network which is electrically associated with the localoscillator tank circuit in such a manner as to simulate across the tankcircuit a reactance of a predetermined sign. The voltage output of thediscriminator is employed to regulate the magnitude 20 of the simulatedreactance across the oscillator tank circuit, and the regulation is suchthat the oscillator frequency is shifted to a predetermined meanoscillator frequency at different settings 4 of the receiver tuningdevice. It is, of course, 1'5' desirable that the corrective oscillatorfrequency shift, at any setting of the tuning device, be substantiallyconstant. In other words, an ideal AFC system would operate in such amanner that the amount of oscillator-frequency correction would i besubstantially constant regardless of` the positioning of the receivertuning mechanism.

Accordingly it may be stated that it is one of the main objects of mypresent invention to provide a device for securing such substantiallyunilSf" form oscillator frequency correction at different settings ofthe tuning device of a superheterodyne receiver employing an AFCcircuit.

Another important object of the invention may be stated to reside in theprovision of a superflui heterodyne receiver of the type utilizing anAFC arrangement, wherein a frequency control tube is electricallyassociated with the local oscillator tank circuit of the receiver insuch a manner as to simulate across the tank circuit a reactance of iffa predetermined sign, and a reactance of a different sign being employedin conjunction with the simulated reactance in such a manner that thepercentage oscillator frequency .shift varies linversely with frequencywhereby there is pro- 5O`5 duced a more constant absolute frequencyshift over the tuning range of the receiver.

Sti-ll other objects of the invention are .generally to improve AFCarrangements for receivers of the superheterodyne type, and moreespecially 55` to provide frequency control arrangements which are notonly eiicient and reliable in operation, but are economically embodiedin superheterodyne receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particu- 5` larity in the appended claims; theinvention it` self, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the drawing in which I haveindicated diagrammatically a circuit organization whereby my inventionmay be carried into effect.

Referring now to the accompanying drawing, wherein like referencecharacters in the two figures designate similar circuit elements, thereis shown in schematic manner in Fig. l a superheterodyne receiver whichemploys an AFC arrangement embodying the present invention. In general,the receiver may be of any conventional superheterodyne type; it willusually em- 2'0 body a signal collector l which feeds a tunable radiofrequency amplifier 2. 'Ihe tunable iirst detector 3 is supplied withamplified signal energy, and oscillations from local oscillator 4 arealso impressed on the detector, or mixer, 3.

The IF energy output of the latter is amplified by an IF amplifier 5,and the output of the amplifier is demodulated by the usual seconddetector network. The latter, and the following audio network, areomitted, because they are 80 well known to those skilled in the art. Itis, also, pointed out that any type of automatic volume controlarrangement may be used to maintain the signal intensity level at thedemodulator input circuit substantially uniform. In this way 3'5 asubstantially uniform signal intensity level is maintained at the inputcircuit of the discriminator.

'Ihe signal circuits 2 and t have the rotors of the variable condensersthereof arranged for 40 mechanical uni-control; the rotor of thevariable condenser t is mechanically coupled to the tun ing adjustingmeans, denoted by the dotted lines l, for the rotors of the signalcircuit condensers.

The tank circuit of the local oscillator i includes 45 y the coil L1shunted by the grounded variable condenser EE. The fixed condenser 9functions as a padder, and the latter acts to maintain the frequency ofthe oscillator tank circuit different from that of the signal circuitsby a predeter- 5o mined constant amount throughout the adjustment rangeof the tuning device l. When the receiver is of the broadcast type andtunable through a range of from 500 to 1500 kc., then the IF may have avalue chosen from a range of from 75 to 480 kc. To secure accuracy intuning, as well as to compensate for oscillator drift, there is employedan AFC arrangement. The AFC may be of the type shown by S. W.,Seeley inhis application Serial No. 45,413, filed Oct. 17, 1935.

The AFC generally comprises a discriminator I functioning to derive adirect current voltage (AFC bias) from the IF energy. The polarity andmagnitude of the AFC bias is dependent on the sense and amount offrequency shift of the IF energy from the assigned frequency. The AFCbias is applied through lead II to an electrode of the frequency controltube I2. The latter functions to produce a predetermined reactive effectacross tank circuit Li-G.

Since the specific construction of the discriminator network is oflittle importance in this case, it will be understood that the networkcan be of any desired type as long as it is capable of converting afrequency shift in IF energy into a direct current voltage change inpolarity and magnitude. For example, the discriminator may compriseoppositely mistuned diode rectifiers, as shown by C. Travis in hisapplication Serial No. 4,793, filed February 14, 1935. Again, the IF-tuned diode rectiers of the aforesaid Seeley application may be employedin the discriminator network if desired. The circuit details of thefrequency control tube and its connection to the oscillator tank circuitwill now be described, since the present invention is embodied in thatnetwork,

The plate I3 of control tube I2 is connected to the positive terminal(+B) of a direct current source through a path which includes the radiofrequency choke coil 8. The cathode il' of control tube I2 is groundedthrough the usual self-bias resistor-shunt capacity network I8. Theplate I3 is connected to the high potential side of oscillator tankcircuit coil L1 through a condenser C3. The platerside of condenser C3is connected to ground through a series path whichV includes resistor Riand the condenser C1. The control grid S9 of tube I2 is connected to thejunction of resistor R1 and condenser C1 through a condenser C2.

There is impressed on the grid IS alternating Y current voltagedeveloped across the path Ri-Ci by oscillator tank current flowingthrough said path. The AFC lead Ii is connected to the grid side ofcondenser C2 through a resistor 20, the function of the latter is toprovide a D. C. path for AFC bias and act as an impedance to audiofrequency considerably higher than the impedance of condenser C1. Byvarying the bias of grid I9 the mutual inductance, or gain, of tube I2is varied. This gain variation, in turn, changes the space current flowto the plate I3, and the current flow through the coil L1.

The control action of tube I2 is produced in the following manner. Thecontrol circuit proper consists of tube I2, the resistors 20 and R1, andthe condensers C1 and C2. A certain alternating voltage, say E. existsbetween ground and the plate of control tube l2. The same voltage existsacross resistor Ri and condenser C1 in series. If the resistor R1 is ahigh resistance, the current through it is nearly in phase vwith voltageE. The

voltage across condenser Ci, however, will lagA is in phase with thegrid voltage. The plate cur-V rent, hence, lags behind the plate voltagenearly 90 degrees.

In an inductance of loW resistance the current also lags Q0 degreesbehind the voltage. Hence, the frequency control tube I2, connected asshown, electrically simulates in shunt across coil L1 an inductance witha small resistance and a condenser C3 in series therewith. This shuntinductance capacity circuit acts to reduce the effective inductance ofcircuit Li-Ii; it increases the frequency of oscillation. The AFC biasapplied to grid I9 acts to vary the gain of the control tube, and'the-magnitude of the simulated shunt inductance, so as to securedesired oscillator frequency correction in response to a frequency shiftin IF energy from the assigned value. The mean bias of grid I9, whichbias is developed by network I8, is so chosen that there will beapproximately equal frequency changes on both sides of the meanoscillator frequency at any setting of the receiver tuning means.However, in actual operation in the past, such uniform oscillatorfrequency correction has not been secured as the variable condenser 6 isadjusted to change the operating frequency of the local oscillator fromone en-d of the tuning range to the other end.

According to the present invention, the magnitude of the condenser C3 isso chosen that this condenser resonates with the simulated shuntinductance to a frequency below the tuning range of the oscillator tankcircuit. In Fig. 2 there is shown the equivalent network with respect tothe oscillator tank circuit coil. It will be observed that theoscillator tank coil L1 has connected in shuntY therewith a seriesv pathVwhich includes the condenser C3 and a reactance designated as L.

The inductive reactance L is the equivalent inductance of tube I2. Inother words, the inductance L represents the simulated inductivereactance which is developed across the tank circuit due to the actionof the frequency control tube I2. The series path Cs-L is resonated to afrequency below the tuning range of the oscillator tank circuit in orderto maintain substantial uniformity of frequency correction of theoscillator tank. circuit throughout the tuning range thereof. Themagnitude of .resistor R1 should be much larger than the reactance ofcondenser vC1. The resonant frequency of Ca-L' decreaseswith decreasingGm of tube I2; hence AFC bias change causes no difficulty.

j I Rlcl L Gm In the above relationship- Gm is that of the control tubeI2. Merely by way of specific example, and not by way of limitation onthe present invention, let'it be assumed that R1=50,000 ohms, C1=20mmf., and Gm=l,000 microhms. In that case L will be equal to 1,000microhenries. For this value of Lgthe condenser C3 should have amagnitude of 42.3 mmf. For these values C3 resonates L to approximately770 kc. Of course, it is assumed for the last named illustration thatthe oscillator tank circuit Iii-6 is tunable through a range offrequencies of approximately 1,000 to 2,2 00 kc., and that the IF is 460kc.; the signal circuits tuning from 540 to 1740 kc.

If the percentage frequency shift were constant, say 10% of theoscillator frequency, the shift would be 100 kc. at a signal frequencyof 540 kc. and 220 kc. at a signal frequency of 1740 kc. 76

With C3 in series equal to 42.3 mmf. the following tabulation shows whathappens:

Assume L1=120 h.

f 1,000 kc. 1,600 kc. 2,200 kc.

6, 280 10, 000 13, 800 3, soo 1, 70u 2, 480 7, 630 12, 100 395 763 880The total shift varies 18% from one end of the range to the other,whereas without C3 it varies 220%. L" stands for the inductance at agiven frequency which would be equal to the combination of L and C3 inseries. This inductance changes with frequency being smaller at the lowfrequencies (nearer LC3 resonance). It is this virtual inductancevarying with frequency which produces the desired effect on shift withfrequency, since a low inductance in shunt with another inductance hasgreater effect.

By resonating the path Ca--L' to a frequency below the tuning range ofthe local oscillator tank circuit several advantages are secured. Thepercentage frequency shift of the oscillator tank circuit variesinversely with oscillator frequency, and, therefore, produces a moreconstant absolute frequency shift. Furthermore, the condenser C3 is inseries with the static capacity of the control tube l2 therebydecreasing the total capacity shunting coil L1. Again, the effectivefrequency variation of the tank circuit is greater, and hence the AFCsystem is more sensitive than with the condenser C3 large, or omitted.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What is claimed is:

1. In combination with a resonant circuit of the type which includesmeans for tuning it over a desired tuning range, a tube connected tosaid circuit to have the cathode to plate impedance of the tube simulatea reactance across the circuit, means for varying the gain of the tubeto adjust the magnitude of said reactance, and a reactance of differentsign from the first reactance in series with the impedance across saidcircuit, said two reactances being resonant to a frequency below saidtuning range.

2. In combination with a resonant circuit of the type which includesmeans for tuning it over a desired tuning range, a tube connected tosaid circuit to have the cathode to plate impedance lof the tubesimulate a reactance across the circuit, means for Varying the gain ofthe tube to adjust the magnitude of said reactance, a reactance ofdifferent sign from the first reactance in series with the impedanceacross said circuit, said two reactances being resonant to a frequencybelow said tuning range, said simulated reactance being inductive, andthe second reactance being capacitative.

3. In combination with a resonant circuit of the type which includesmeans for tuning it over a desired tuning range, a tube connected tosaid circuit to have the cathode to plate impedance of the tube simulatea reactance across the circuit, means for varying the gain of the tubeto adjust the magnitude of said reactance, and a reactance of differentsign from the first reactance in series with the impedance across saidcircuit, said two reactances being resonant to a frequency below saidtuning range, said gain varying means adjusting said magnitude when thecircuit frequency departs from predetermined frequency values of saidrange.

4. In combination with a coil tuned to a desired frequency, a tubehaving connections thereto to have the cathode to plate impedance of thetube produce an inductance effect across the coil, a condenser in serieswith said impedance across the coil, said condenser resonating saidinductance to a frequency below said desired frequency.

5. In combination with a resonant circuit of the type including a coiland a condenser in shunt therewith, said circuit being tunable through adesired frequency range, an electron discharge tube including at least acathode, control grid and an anode, means for impressing the outputcurrent of said tube on said resonant circuit, a circuit elementconnected to said resonant circuit whereby the voltage across saidcircuit element is substantially in quadrature with the voltage acrosssaid resonant circuit, means for applying said quadrature voltage tosaid control grid whereby the effective reactance of the coil of saidresonant circuit is decreased by virtue of a simulated inductivereactance produced across said coil, and a condenser effectivelyconnected in series with said simulated inductance across said coil,said condenser and simulated inductance being resonant to a frequencybelow the tuning range of said resonant circuit.

6. In a superheterodyne receiver of the type including an automaticfrequency control circuit,

said automatic frequency control circuit being of the type whichincludes a frequency control tube having input and output electrodeselectrically coupled with the local oscillator tank circuit of thereceiver to produce a simulated inductive reactance across the tankcircuit, a condenser connected between the output electrode of saidcontrol tube and the high alternating potential side of said tankcircuit whereby the condenser is effectively in series with saidsimulated reactance across the tank circuit, and said condenser andsimulated reactance being resonant to a frequency below the lowestfrequency of said tank circuit.

7. In a superheterodyne receiver of the type including a localoscillator network provided with a tank circuit tunable over arelatively wide frequency range, an intermediate frequency network, anelectron discharge tube having input and output connections to the tankcircuit such that the cathode to plate impedance of the tube acts as aninductance across the tank circuit, and a discriminator, responsive toshifts in the intermediate frequency energy from an assigned frequency,for controlling the gain of said tube in a sense to cause the inductanceto correct the frequency of the tank circuit and maintain said assignedfrequency value; the improvement which comprises a condenser in serieswith said cathode to plate impedance across said tank circuit, and saidcondenser resonating the said inductance to a frequency below saidfrequency range whereby said correction is substantially uniform oversaid range.

DUDLEY E. FOSTER.

